Polyester resins have found widespread use in varied applications. Polyester resins, such as polyethylene terephthalate, are used in films, including photographic film, in fibers, and in food and beverage containers. Various methods have been disclosed for the depolymerization of such resins into their component monomers, such as ethylene glycol and terephthalic acid, or derivatives thereof, so that they could be reused.
Some of these methods are described in such patents as U.S. Pat. Nos. 3,037,050, 3,321,510, 3,884,850, 3,907,868, 4,163,860, 4,578,502, 4,620,032, 4,876,378 and 5,095,145, and in European Published Patent Application 0 484 963 published May 13, 1992.
A particularly useful technique for recovering scrap polyester is described in a series of patent that begins with Naujokas et al. U.S. Pat. No. 5,051,528. This patent describes a process of recovering ethylene glycol and dimethyl terephthalate from polyethylene terephthalate scrap resins by dissolving the polyester resin in oligomers of the same monomers as present in the polyester, passing super-heated methanol through the solution and recovering ethylene glycol and dimethyl terephthalate.
Gamble et al. U.S. Pat. No. 5,298,530, issued Mar. 29, 1994 describes improvements in this process. In the improved process scrap resin is combined with melt from the reactor in a dissolver, and the dissolver melt is transferred to the reactor for contact with super-heated methanol. In the reactor, polymers and oligomers are further depolymerized into component glycol and ester monomers, which are then recovered.
Further improvements and variations of this process are described in Gamble et al. U.S. Pat. No. 5,393,916 issued Feb. 28, 1995 and in Toot et al. U.S. Pat. No. 5,414,022 issued May 9, 1995.
The processes described in these patents reverse the polymerization reaction by which the polyethylene terephthalate is formed by depolymerizing polyethylene terephthalate (PET) to dimethyl terephthalate (DMT) and ethylene glycol (EG). DMT and EG are recovered and reused. If the depolymerization reaction does not go to completion, an intermediate half ester, methylhydroxyethyl terephthalate (MHET), is present in the product stream. MHET is relatively soluble in methanol and will remain in solution when DMT is separated from other components in the product stream by crystallization, thus reducing the yield of DMT from that theoretically possible. Furthermore, when the liquid phase, containing methanol and EG, is separated from the DMT crystals, any MHET present will go with the methanol, where it will complicate the recovery of methanol and EG.
Thus, it would be desirable to have a process for converting MHET to DMT during the recovery of the latter, so as to increase the yield of DMT and to eliminate the need to separate MHET in subsequent recovery operations.